One-click unsubscribe is a message-level feature
One-click unsubscribe is implemented in email headers and the sender's web endpoint, not in DNS. A domain can pass SPF, DKIM, and DMARC while a marketing message still lacks the unsubscribe controls expected for subscribed mail.
The List-Unsubscribe header identifies an HTTPS endpoint or mailto method. Standards-based one-click behavior uses List-Unsubscribe-Post so the receiver can send a POST request without asking the user to visit a preference page.
What the sending platform must do
Use an HTTPS endpoint that accepts the one-click POST and immediately records the request. Do not require login, extra confirmation, or a landing-page interaction for the header action. Keep a clear unsubscribe link in the visible message body as well.
Use your email service's supported implementation rather than hand-editing headers in a template. The platform must generate recipient-specific tokens safely and suppress future subscribed mail within the required operational window.
How to test it
Send a campaign test to a mailbox you control, view the original source, and confirm both List-Unsubscribe and List-Unsubscribe-Post are present where required. Test the provider's unsubscribe workflow with a non-production recipient and verify that suppression is recorded.